Chemokines are a group of 7- to 14-kd peptides that play an important role in orchestrating leukocyte recruitment and migration during inflammation, and therefore represent an important target for anti-inflammatory therapies (Wells et al., 2006). They act by binding to seven-transmembrane, G protein-coupled receptors, the chemokine receptors. The chemokine system is complex, with about 50 chemokines and 20 chemokine receptors identified in humans, often acting with redundancy, making selection of specific antagonists difficult (Gerard and Rollins, 2001). Genetic knockout strategies have confirmed the importance of chemokines as regulators of immune function, but the deletion of specific chemokines has led to only specific and relatively mild defects in the inflammatory response further emphasizing the complex redundancy of the system. Selectivity is crucial for use of chemokine receptor antagonists in systemic diseases where a single chemokine-receptor system is implicated such as atheroscelorsis where the macrophage/monocyte system is the major player in order to allow a subtle and specific control over immune function (Weisberg et al., 2006; Feria and Diaz Gonzalez et al., 2006).
Many ocular conditions are characterized by inappropriate migration and infiltration of cells such as leukocytes and endothelial cells into the eye with deleterious effects to ocular structures (Wallace et al., 2004). Chemokines have been identified in such diseases and misregulation of the chemokine system is apparent in corneal graft rejection, diabetic retinopathy, age-related macular degeneration (ARMD), chronic inflammatory diseases such as uveitis, dry eye etc. Mice lacking CCR2 or MCP-1 develop features of ARMD with age, including drusen deposits, choroidal neovascularization and photoreceptor atrophy indicating a crucial role for this chemokine and its receptor signaling (Amabati et al., 2003). Thus CCR2 receptor-specific inhibitor might have potential therapeutic benefit in ocular diseases like ARMD. In contrast, various human and animal studies have identified several chemokines in different forms of uveitis, produced both by resident and infiltrating cells, that strongly suggests a prominent role for these molecules in its pathogenesis. Studies in rat and mice models of uveitis have demonstrated up-regulation of monocyte chemoattractant protein-1 (MCP-1), macrophage inflammatory protein-1 (MIP-1), RANTES, stromal derived factor-1 (SDF-1) which are powerful chemoattractants for monocytes and T-cells (Fang et al., 2004; Keino et al., 2003). Similar findings have been reported in peripheral blood mononuclear cells in patients with acute anterior uveitis (AAU), the most common form of human uveitis (Klitgaard et al., 2004). MCP-1 knockout mice and CCR5 knockout mice show reduced endotoxin-induced uveitis, which is the animal model for AAU (Takeuchi et al., 2005; Tuallion et al., 2002). It has also been demonstrated that blocking the chemokine system upstream with the use of NF-κB blockers significantly attenuates experimental AAU in rats (Yang et al., 2005). Blockage of NF-κB results in transcriptional inhibition of multiple chemokines. Given the complexity of pathogenesis in uveitis it is unlikely that a selective inhibition of a chemokine receptor in monotherapy will offer therapeutic benefit. A similar role of multiple chemokines have been shown to be correlated with clinical stage of disease in diabetic retinopathy and dry eye (Meleth et al., 2005; Yamagami et al., 2005). In these ocular diseases the use of broad spectrum chemokine receptor inhibitor which inhibits the function of a wide range of chemokines may be beneficial.
The first broad spectrum chemokine inhibitor (BSCI) to be reported was termed Peptide 3, which was derived from the sequence of human chemokine MCP-1 and was shown to block the migration of monocytes in response to MCP-1, MIP-1, RANTES and SDF-1 (Reckless and Grainger. 1999). A cyclic retro inverse analogue of Peptide 3, constructed of D-amino acids in the reverse sequence, called NR58-3.14.3 was observed to be a more potent chemokine inhibitor (Beech et al., 2001). NR58-3.14.3 has been used to test for anti-inflammatory activities in animal models of atherosclerosis, lung inflammation, irritable bowel syndrome etc (Beech et al., 2001; Grainger and Reckless. 2003; Tokuyama et al., 2005). However there are several disadvantages to using these BSCI as a long-term therapeutic strategy. The known BSCIs which are peptides which have relatively low potency, poor pharmacokinetics, and are unstable in vivo. In addition, systemic use of broad spectrum chemokine receptor inhibitors could potentially lead to deleterious side effects due to their systemic anti-inflammatory activity. However in ocular diseases, a local or topical application would prevent the broad spectrum inhibitor to be taken up systemically. Identification of a small molecule inhibitor of several chemokine receptors could be very useful for treatment of inflammatory ocular diseases. Given the evidence for the role of multiple chemokines in several ocular diseases and these results, we propose that the use of small and large molecule broad spectrum chemokine receptor inhibitors will have utility in the local treatment of ocular inflammatory diseases including, but not limited to, uveitis, dry eye, diabetic retinopathy, allergic eye disease and proliferative retinopathies. Manipulation of multiple chemokines therefore represents a novel therapeutic approach in treating ocular diseases.
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